Ceramic filter elements have an asymmetric structure in which thin membrane layers are applied, with one or more intermediate layers, to a porous ceramic support. The porous ceramic support determines the external shape and the mechanical stability. Customary embodiments are:
Tubes
Tubes are produced by extrusion. The most widely used dimensions are:
1-channel tubes: external diameter (ED)/internal diameter (ID)=10 mm/7 mm,
7-channel tubes ED/channel diameter (CD)=25 mm/7 mm,
19-channel tubes ED/CD=25 mm/3.5 mm or larger.
The length of the industrially used filter elements is up to 1.00 m, in some cases up to 1.20 m.
Honeycombs
An extruded body consisting of a very large number of channels (round or polygonal) having small wall thicknesses is referred to as a honeycomb. In such a body, large membrane areas can be accommodated in a very small space. However, the wall thickness of the channel is too small for removing permeate to the outside. Individual channels are therefore closed in a reciprocal manner and used for permeate removal (EP 0 306 350, EP 0 433 582, EP 0 923 983 and WO 00/50156).
Plates
Plates are produced by pressing, casting or extrusion. The thickness of the plates is several millimeters. The plates can be produced with a profile. Feed, retentate and permeate channels are formed by stacking the plates.
Discs
Discs are produced by film casting. The thickness of the film is in the range of 0.5 mm-3 mm. The film pockets can be produced by laminating the discs (DE 4 330 163).
Capillaries
Capillaries having an external diameter of 10 mm to about 1 mm can be produced by extrusion. By approximately maintaining the ratio ID/ED=7/10, a high internal pressure resistance comparable with that of single-channel tubes is obtained in the case of the capillaries after firing. The capillaries are rigid and can be individually handled.
A special method is melt extrusion, in which the thermoplastic organic binder is used and the shaping takes place at elevated temperature (DE 694 00 874).
Hollow Fibers
The term hollow fibers is used in the case of external diameters of less than 1 mm. This requires a gas to flow through the capillaries during extrusion in order to prevent a collapse of the soft, plastic material (WO 99/22852). Very small diameters can be produced by spinning (JP 05/221752, JP 02/091221). For this purpose, a polymer is filled with a ceramic powder and this is spun as a hollow fiber or a polymer is dissolved in a solvent and a ceramic powder is suspended and spun in a precipitating bath. After firing, the hollow fibers retain a certain elasticity. They can no longer be processed as individual elements.
Ceramic filter elements are arranged parallel in housings. The result is a module which contains connections for feed and discharge (retentate) and a connection for the filtrate (permeate) and optionally a further one for flushing liquid (sweep). In the case of multichannel tubes, the membrane is applied to the inside of the channel and filtration is effected from the inside outwards. In the case of single-channel tubes, capillaries and hollow fibers, there are examples where the membrane is applied on the outside or inside. Where the diameter of the capillaries or hollow fibers is too small, an inner coating is no longer possible.
The module form, in particular the method of fixing the membrane elements, the flow conditions and the size, depends on the type of support and the specific separation method.
In tube modules, the tubes are installed individually and sealed by sealing rings or special sealing caps (EP 0 270 051, DE 198 46 041, EP 0 270 051) at the ends with the housing. Methods in which metallic connectors are adhesively bonded or soldered to the ends of the tubes have also been described (DE 4 131 407). In the case of capillaries and hollow fibers, this leads to an unacceptably complicated assembly or, because of the flexibility in the case of hollow fibers, is not possible at all. Methods in which a relatively large number of capillaries or hollow fibers are processed to give bundles are required here.
With the use of honeycombs, modules are described in which holes or slots are introduced laterally, which holes or slots open the permeate channel and permit lateral emergence of permeate (U.S. Pat. No. 5,855,781, EP 1 060 784).
Filter pockets are preferably provided with a central hole and threaded onto a permeate collecting tube. This gives stacks which are combined to form modules (DE 4 330 163).
In contrast to these modules, the present invention describes a module using ceramic capillaries. The following technical solutions are known in this area:
In EP 0 938 921, bundles of capillaries consisting of glass-like carbon membranes are placed in a mould and cast in a resin which is filled with a solid, the resin being subjected to an ultrasonic treatment.
EP 0 841 086 describes a module comprising hollow fibers which contains filaments (rods) having a diameter of from 0.06 mm to 3 mm, in the ratio of 0.5 times to 5 times the number of hollow fibers, for achieving mechanical stability, the hollow fibers and the rods being held at the ends by perforated plates. In US 01/0035374, hollow fiber bundles are stabilized by winding the filaments in a spiral around the bundles. Comparable mechanical stabilization is not necessary in the case of ceramic capillaries according to the present invention.
EP 0 941 759 describes a method for the production of an exchanger, in which a bundle of exchanger tubes is placed in a mould and then filled with a ceramic slip, dried and then sintered to give a plate. In a further step, an end plate is moulded in the same manner. Collecting tubes for feed and discharge are introduced into the first plate and the end plate, parallel to the tubes. It is mentioned that such an arrangement can be used for membrane modules, no capillaries being used and no application-specific distance between the tubes being established.
DE 4 133 250 claims a method for the production of membrane tube bundles, in which a uniform distance between the membrane tubes is established by widening of the ends to form regular polygons and connection in a substance-impermeable manner by adhesive bonding or welding. In contrast, the capillaries in the present invention are not widened at the ends.
EP 0 092 839 uses a nonporous end plate through which holes are drilled in which porous tubes are fastened by means of special connections. The fastening is effected by an enamel, glass, ceramic, carbon, cement or metal. In WO 01/87469, a completely ceramic module is assembled by joining ceramic supports to a housing comprising perforated end plates and cylindrical outer casing. In all these cases, the perforated plates serve for mechanical fixing of the tubes or capillaries. The tubes are a fixed distance apart, which however bears no relationship to the separation method, in particular no relationship to the amount of permeate.
JP 61/004509 uses an aqueous slip of a mixture of glass powder and ceramic powder with an adapted coefficient of expansion for sealing the ends of the porous glass membrane tubes. JP 57166244 describes the sealing of small tubes with a sealing material, e.g. glass, with the result that an air-tight end plate forms. In both cases, sealing of the ends of the capillaries and mechanical fixing are achieved. However, no application-related, defined distance is established.
U.S. Pat. No. 4,296,052, U.S. Pat. No. 4,224,386 and U.S. Pat. No. 4,219,613 describe the production of perforated plates for hollow fiber batteries, which consist of an upper helium-tight region and a porous region underneath and through which a large number of inorganic hollow fibers pass. The hollow fiber walls serve as diaphragms through which no material transport takes place and hence furthermore no defined distance between the hollow fibers is required.
It is the object of the invention to eliminate the described deficiencies of the prior art in the case of a separation module with optimum utilization of space based on the parameters of the specific application.